1. Technical Field
The present invention relates to methods of polishing compound semiconductor substrates, to the compound semiconductor substrates, to methods of manufacturing compound semiconductor epitaxial substrates, and to the compound semiconductor epitaxial substrates; more particularly the invention relates to compound-semiconductor-substrate polishing methods that reduce oxygen superficially present on the substrate, to the compound semiconductor substrates, to compound-semiconductor epitaxial-substrate manufacturing methods, and to the compound semiconductor epitaxial substrates.
2. Description of the Related Art
Compound semiconductor substrates have been widely used for semiconductor lasers, light emitting diodes (LEDs), and high-speed devices because they have luminescent properties and high electron mobility. Ordinarily, in compound-semiconductor-substrate manufacturing procedures polish is carried out. Methods of polishing compound semiconductor substrates are disclosed, for example, in Japanese Examined Pat. App. Pub. No. H07-67666, in Japanese Unexamined Pat. App. Pub. Nos. 2003-86553, 2001-144056, 2002-25954 and 2003-100671, and in Japanese Pat. Nos. 3680556 and 3551229.
Just-noted Japanese Examined Pat. App. Pub. No. H07-67666 discloses a technique whereby III-V compound semiconductor substrates are polished employing a polishing agent having as its principal components chloroisocyanuric acid, an alkali metal phosphate, and hydrosulfate. In Pub. No. H07-67666, polishing is done by superficially oxidizing a compound semiconductor substrate by means of the chloroisocyanuric acid contained in the polishing agent, and reductively removing the oxide by means of an alkali metal salt. An oxide film ends up being left on the substrate surface, however, because the chloroisocyanuric acid contained in the polishing agent is strongly oxidizing. Moreover, clearing away the oxide film in the post-polishing stages leading up to the final cleaning process is exceedingly difficult.
Disclosed in above-referenced Japanese Unexamined Pat. App. Pub. 2003-86553 is a technique of cleaning semiconductor crystal wafers with ultrapure water having a temperature of 15° C. or lower, directly after the end of a polishing process using an aqueous hypochlorite solution as the polishing solution. Because the polishing solution employed is a strongly oxidative aqueous hypochlorite solution having a pH of 8 to 10, superficial oxidation of the semiconductor-crystal wafer proceeds in the interval from after the end of the polishing until in the purified water the strongly oxidative polishing solution clinging to the wafer surface is replaced with the pure water. As a consequence, an oxide film forms thickly on the semiconductor-crystal wafer surface.
In above-referenced Japanese Unexamined Pat. App. Pub. 2001-144056, a technique of polishing semiconductor crystal wafers and, directly post-polish-completion, cleaning the wafers with an acidic aqueous solution having of pH 3 to 5 is disclosed. Even though the polishing solution is neutralized in the acidic aqueous solution, oxidizing potential remains following polish completion, because the polishing solution employed is a strongly oxidative aqueous hypochlorite solution having a pH of 8 to 10. Inasmuch as oxidation of the semiconductor-crystal wafer surface consequently proceeds even after polishing is completed, an oxide film forms thickly on the wafer surface.
Above-referenced Japanese Unexamined Pat. App. Pub. Nos. 2002-25954 and 2003-100671, and Pat. No. 3680556 disclose techniques of polishing semiconductor crystal wafers using as the polishing solution a mixture made blending a surfactant into a base polishing liquid. However, because oxide-film buildup cannot be sufficiently cleared away with a surfactant, an oxide film forms thickly on the wafer surface.
Aforementioned Japanese Pat. No. 3551229 discloses a technique whereby, when polishing of the front side of a semiconductor substrate is completed, an aqueous solution containing both a thickener and an oxidant is used to put a halt to the polishing reactions. However, the fact that the aqueous solution for stopping the polishing reactions contains an oxidant and the fact that the aqueous solution is employed post-polish mean that, with the oxidation reaction is not being halted, formation of oxide film on the substrate surface cannot be kept adequately under control.
With the techniques in above-referenced Japanese Examined Pat. App. Pub. No. H07-67666, in Japanese Unexamined Pat. App. Pub. Nos. 2003-86553, 2001-144056, 2002-25954 and 2003-100671, and in Japanese Pat. Nos. 3680556 and 3551229, polishing compound semiconductor substrates leads to an oxide film being formed superficially on the substrate.
For this reason, in situations in which to fabricate stacked devices, epitaxial layers are grown on a compound semiconductor substrate having an oxide film on its front side, the oxygen in the oxide film acts as impurities, wherein electrons passed into the devices to operate them end up being trapped by the oxygen present between the compound semiconductor substrate and the epitaxial layers. A consequent problem is that the number of electrons that reach the active layers drops, adversely affecting the device properties. A further problem is that a heavy presence of oxide film on the compound semiconductor substrate worsens the morphology of (the growth striations and defects in) epitaxial layers built onto the oxide film.